The Urban Dictionary T-Shirt

(Using Supernatural logic here)
When Cain and Abel were born, Lucifer had already planned on taking Abel. After they made their sacrifices to God, Lucifer appeared to Cain. He told him how he planned to take his brother, but Cain would not let him. Lucifer said if Cain killed Abel, Abel would go to heaven. But, Cain would be stuck on Earth with a mark (later known as the Mark of Cain). He could never age, and he could never love. He soon became a monster, killing for the joy of the kill.

Cain warned Dean of the trouble the Mark of Cain would bring, but Dean took it on anyway. He accepted the heavy burden in order to kill the knight of hell known as Abadon.